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PO"Selecting" or "Wrapping" an IQueryable so that it is still queryable
 

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  1. PO"Selecting" or "Wrapping" an IQueryable so that it is still queryable
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    <p>I have a Class / API that uses an <code>IQueryable&lt;FirstClass&gt;</code> data source however I wish to expose an <code>IQueryable&lt;SecondClass&gt;</code>, where <code>SecondClass</code> is a wrapper class for <code>FirstClass</code> that exposes nearly identical properties, however for various reasons needs to inherit from an unrelated base class. For example:</p> <pre><code>// My API void IQueryable&lt;SecondClass&gt; GetCurrentRecords() { return from row in dataSource /* Linq query */ select new SecondClass(row); } // User of my API var results = GetCurrentRecords().Where(row =&gt; row.Owner = "Mike"); </code></pre> <p>Now I can make the above compile simply by using <code>AsQueryable</code> however I want to expose a "true" <code>IQueryable</code> that efficiently queries the database based on the API users query.</p> <p>I know that this isn't trivial (my wrapper IQueryable implementation needs to understand the relationship between the properties of <code>SecondClass</code> and <code>FirstClass</code>), and that it has nothing to do with the <code>Select</code> function, but it <em>seems</em> like it should be possible.</p> <p>How do I do this?</p> <p><strong>Note:</strong> I know that instead my API could just expose <code>FirstClass</code> along with a helper method to convert <code>FirstClass</code> to <code>SecondClass</code> for when the API user is "done" creating their query, but it feels messy and I don't like the idea of exposing my generated classes in this way. Also I'd like to know how to do the above anyway just from a purely academic standpoint.</p>
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    1. USJustin
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      1. USMark Coleman
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    1. COThis question is meaningless without the code you haven't included. L2E already does this for `ObjectQuery<T>`. Your own code (producing `dataSource` might not, but you haven't shown that.
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      1. PO"Selecting" or "Wrapping" an IQueryable so that it is still queryable
      1. USCraig Stuntz
    2. CO@Craig I don't follow - what is it that you need to see? `dataSource` is just a generated class that inherits from `ObjectQuery<FirstClass>`. Are you saying that this should *already work*? What is this magic!?!
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      1. USJustin
    3. COIf `dataSource` is an `ObjectQuery<FirstClass>` then your query will fail at runtime since L2E doesn't support parameterized constructors. But the "magic" is [LINQ to Entities projections,](http://blogs.teamb.com/craigstuntz/2009/12/31/38500/) which already return `IQueryable<T>` and support further composition with SQL support.
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      1. USCraig Stuntz
 

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